You have two options for installing GRUB; either to /dev/sda (no number), which would put GRUB on the MBR of your hard disk. This puts GRUB in control of the dual booting, so it will show a menu at boot to choose which operating system to load. Or you can install GRUB to the partition on which you install Linux Mint. You would then have to install another boot manager to your MBR from Windows, like EasyBCD, so that this program gives you a way to choose which operating system to boot.
Linux Mint 14 Nadia / 64-bit / Cinnamon